Corrosion in Commercial Heating and Hot Water Systems

Corrosion is one of the most common reasons commercial heating and hot water systems fail early, and one of the most preventable. This guide brings together everything you need to know: what causes corrosion, how to test for it, which materials resist it best, and how to control it in both hot water cylinders and closed heating or chilled water loops.

What causes corrosion in commercial hot water systems?

Most metals corrode over time when they’re in contact with water, whether that’s rusting iron or the slower deterioration of copper, lead, aluminium and zinc. In a heating or hot water system, where metal appliances and pipework are in near-constant contact with water, this becomes a real business risk rather than a slow background process. No single factor causes corrosion on its own, but water softness is one of the biggest. Water that’s passed through limestone and chalk, common in the South East of the UK, picks up calcium and magnesium and is classed as hard above 280ppm. Water that’s passed through hard igneous rock, as it does across Scotland, the North West and South West of England and Western Wales, stays naturally purer, usually under 100ppm. That soft water has a low pH, low dissolved solids and almost no buffering capacity, all of which make it more corrosive.

pH is a big part of the picture. It measures how acidic or alkaline the water is, and the ideal pH for a commercial hot water system sits just above 7. Below that, water is acidic, and acids release hydrogen ions that oxidise metal and speed up corrosion, so the lower the pH, the more aggressive the attack tends to be. Temperature plays a role too: the 60°C commercial hot water systems need to reach as standard already accelerates chemical reactions, and it’s not unusual for parts of a system to hit 70°C, which pushes the corrosion rate up further still.

The main types of corrosion to know

Corrosion needs anodic and cathodic sites to form, and small variations in a metal’s surface, from manufacturing stress to impurities to grain size, create them. When those sites move around the surface, you get a fairly even, uniform corrosion, usually visible as general surface oxidation. When they stay fixed in one place, you get the more serious, localised forms of corrosion.

Pitting. one of the most destructive forms, happens when static anodic and cathodic sites, combined with low water velocity, create a small pit on the metal surface. The water trapped inside becomes isolated and increasingly acidic over time, which makes the pit grow faster the longer it’s left.

Leaching (dezincification). is the selective corrosion of one element out of an alloy, most often zinc leaching out of brass fittings. The copper left behind changes colour and loses mechanical strength, which is why cheaper brass valves and fittings can fracture and leak, particularly where the water has low pH, low velocity or high chlorine content.

Galvanic corrosion. occurs when two different metals, such as copper and stainless steel, come into contact with each other and water, which is common in refurbishment projects mixing old and new pipework. The water acts as an electrolyte, and the less noble (more anodic) metal corrodes faster while the nobler (cathodic) metal is protected. The further apart the two metals sit on the galvanic series, and the closer they are physically, the worse the corrosion at that contact point.

Erosion corrosion. is usually a design problem rather than a water quality one. Oversized pumps push water through a system at high velocity, and any suspended solids in that fast-moving water wear away at the metal, particularly where the flow changes direction or hits an obstruction. It shows up as smooth, grooved damage that follows the direction of flow.

Chemical-driven corrosion. comes from substances like chlorine, chloramine and dissolved oxygen in the water, all of which encourage metal to lose electrons. The balance between chloride and sulphate (the CSMR) matters too: sulphates help form a protective film, while chlorides break it down and can accelerate lead leaching from older materials.

Corrosion control in closed heating and chilled water systems

Corrosion isn’t only a hot water cylinder problem. Closed loop heating and chilled water (CHW) systems, the pipework that circulates low temperature hot water (LTHW) around a building for space heating, or chilled water for cooling, face many of the same risks, plus a few of their own. Because these systems are sealed, dissolved oxygen ingress at pump seals, expansion vessels or during maintenance is often the main driver of corrosion, alongside the same galvanic risk wherever dissimilar metals meet, such as steel radiators connected to copper pipework.

Controlling corrosion in a sealed system usually comes down to three things: water treatment, monitoring and good design. Chemical inhibitors are dosed into the system to slow corrosion and limescale formation, and should be checked regularly, since their effectiveness fades over time and needs topping up. Increasingly, buildings use real time or “intelligent” corrosion monitoring, electronic sensors or coupons that track corrosion rates or water chemistry continuously, rather than relying on periodic manual sampling, which makes it much easier to catch a problem before it causes a leak. On the design side, minimising air ingress points, protecting thin wall steel components (which corrode faster than heavier-gauge pipework once inhibitor levels drop), and avoiding unnecessary dissimilar metal contact all reduce the underlying risk before you even get to water treatment.

Testing your water for corrosivity

If your building sits in a soft water area, small leaks in metal plumbing components are worth taking seriously, since they’re often an early sign of a bigger corrosion problem. Testing for pH, calcium concentration, hardness, dissolved solids and temperature, along with dissolved lead and copper, is relatively straightforward and worth doing as standard.

The Langelier Saturation Index (LSI) is the longest-established measure, developed to show whether water is likely to be scale-forming or scale-dissolving. A positive LSI suggests non-corrosive water, a negative one suggests corrosive water. It wasn’t designed for naturally soft water though, and it rests on the assumption that a thin, even layer of calcium carbonate scale protects against corrosion, an assumption with little real scientific backing. In a real commercial system, scale rarely forms evenly. It tends to build up on the hottest parts of a heater, and the crevices it creates can actually drive more localised corrosion, similar to pitting, rather than preventing it.

The Ryznar Stability Index (RSI) gives a more reliable picture, factoring in pH, conductivity, calcium ions, bicarbonate and temperature. An RSI of 6.0 to 7.0 suggests a little scale and some risk of corrosion; 7.0 to 7.5 means significant corrosion; 7.5 to 9.0 is heavy corrosion; and anything above 9.0 is considered intolerable. Both LSI and RSI focus on scale, so testing directly for metal concentrations, particularly copper, iron, manganese and aluminium, gives a more direct read on whether corrosion is already happening. One caveat: in open systems, such as those with instantaneous water heaters, corrosion by-products get flushed out quickly, so a single test can miss an ongoing problem, which is why regular testing matters more in soft water areas. UK water suppliers can usually provide local water quality data based on postcode, and independent testing is available too. Adveco’s engineers can help with water assessment as part of the design process.

Choosing corrosion-resistant materials

Once you understand the risk, material choice is the biggest lever you have. Dezincification Resistant Brass (DZR), made by adding tin to standard brass, resists the leaching problem described earlier and is marked CR or DZR on UK fittings. For tanks and cylinders, the two main corrosion-resistant options are glass-lined steel and stainless steel.

Glass-lined tanks work by bonding a first coat of glass to the steel shell, then building up further chemically resistant layers on top, which makes them smooth, easy to clean and popular in harder water areas. The risk is that the lining can develop microscopic cracks, exposing the steel underneath, and corrosion factors like fluoride will attack a glass lining at any temperature. Galvanic protection for these tanks usually comes from a magnesium sacrificial anode, which corrodes in place of the steel shell and needs regular checking and replacing, or a powered titanium anode, which produces a low protective current without corroding itself. Both approaches lose effectiveness in soft water, where low conductivity reduces how well the anode can do its job. In fact, in soft water areas, an anode that looks in excellent condition on inspection can be a warning sign in itself, since it may mean the anode has stopped working and the tank itself is corroding instead.

Stainless steel avoids this problem altogether because its corrosion resistance is built into the alloy, not a coating. That’s why commercial systems in Scotland, the South West and North West of England and West Wales, where water is naturally soft, typically specify stainless steel rather than glass-lined alternatives. It’s also the standard choice for condensing water heater heat exchangers, where flue gas condensate is acidic enough to damage ordinary steel or copper quickly. The upfront cost is higher, but in soft water areas in particular, it usually works out cheaper over the system’s lifetime than replacing a corroded glass-lined tank.

The cost of getting it wrong

Corrosion failures are expensive in more ways than one. Beyond the cost of replacing a failed cylinder, heat exchanger or section of pipework, there’s the disruption of losing hot water or heating in a building that depends on it, and the risk that a slow leak goes unnoticed until it causes real damage. Oversizing a system is a surprisingly common contributor: bigger pumps than necessary don’t just add capital and running costs, they also drive the high-velocity flow that causes erosion corrosion, so correct sizing is as much a corrosion control measure as it is an efficiency one.

Regular servicing matters just as much as good design and material choice. Descaling, flushing sediment, checking anodes and testing water quality all extend an appliance’s working life. Skip them in a soft water area and a system that should last years can fail within months. Getting the sizing, materials and maintenance schedule right from the start is what keeps a commercial hot water or heating system running safely, efficiently and cost-effectively for its full working life.

Frequently asked questions

What is galvanic corrosion in a water heater. it happens when two different metals, such as copper and stainless steel, are connected in the same water-filled system. The less noble metal corrodes faster than it would alone, while the more noble metal is protected, with the effect strongest close to where the two metals meet.

Does magnesium chloride accelerate copper corrosion in hot water. chloride ions generally do increase the corrosion risk for copper, since they interfere with the protective passive layer that would otherwise form on the metal’s surface. The exact effect depends on the water’s overall chemistry, including its pH and sulphate levels, so a full water test gives a clearer picture than looking at any one compound alone.

What’s the difference between a glass-lined and a stainless steel water heater. a glass-lined heater is a steel tank coated in a layer of glass for protection, which is cheaper upfront but vulnerable if the coating chips or cracks, especially in soft water. A stainless steel heater has corrosion resistance built into the metal itself, costs more upfront, and generally lasts longer, particularly in soft water areas.

How do I control corrosion in a closed heating or chilled water system. through a combination of chemical inhibitor dosing, checked and topped up regularly, ongoing water quality or corrosion rate monitoring, and design choices that limit oxygen ingress and dissimilar metal contact.

Every building’s water is different, and the right approach to corrosion control depends on your local water quality, system design and materials. Adveco’s engineers can help assess your water and specify the right appliances and protection for your project.